Many computer systems such as those systems used by scientists and airline reservations systems must process and store large amounts of data. Loss of data caused by data storage medium storage failure or damage could take many man years to replace, if replacable at all. Thus, many of these users are demanding more fault tolerant data storage systems.
As the capacity of disk drives and other such direct access rotating data storage media has been increased, there has been a decrease in the price and size of such devices. One solution for providing the required fault tolerance is to provide a data mirroring feature wherein each data storage medium has a duplicate storage medium or mirror image operating in parallel with the primary device. Such a system allows the user to make a second copy on a second storage medium, of every data file as it is written to the primary storage medium. 0n such a system, data may be read from either the main data storage medium or the "mirror" medium.
Although providing a data "mirror" medium does result in additional system costs and other associated overhead, the resulting benefits of data redundancy often far outweigh the costs. Such costs could be further counterbalanced if data-mirroring systems could provide additional system performance during "read" operations which typically comprise 70% of the requests from the host system.